This invention relates to acoustic transducers and, more particularly, to a multipole loudspeaker implementation which can adaptively reduce the lateral transmission of an acoustic signal.
Handsfree communication devices are used extensively where users are required to either communicate for long periods of time or where they require the use of their hands while communicating with others. In such devices, a loudspeaker is used for making received signals audible to a user of the device. As a result, the transmission of audio signals to listeners other than the intended listener is a frequent occurrence and this often results in a compromising of the user""s privacy and disruption of his neighbours.
Conventional directional speakers depend on speaker geometry, i.e. cones, horns or reflecting surfaces, and are only directional in a frequency range having corresponding sound wavelengths which are smaller than or comparable to the characteristic size of the speaker. The characteristic size of a speaker is considered to be the largest dimension of the speaker, i.e. either its acoustic driver""s largest dimension or the largest dimension of an associated speaker housing. For example, a 334 Hertz (wavelength=1 meter) audio signal would require a conventional directional speaker having a characteristic size of about one meter to provide substantial directionality. This would clearly not be practical in most situations.
Multipole loudspeakers, on the other hand, are directional sound sources which can radiate sound preferably into a specific spatial region exteriorly of the speaker without the use of reflecting surfaces and, more specifically, are able to do so in a frequency range corresponding to sound wavelengths (lambda) which are much greater than the characteristic size of the speaker. Advantageously, privacy is further enhanced as the sound pressure level of multipole speakers attenuates at a faster rate than for regular loudspeakers as the distance from them is increased. For example, in conventional (monopole) speakers, the sound pressure level attenuates at a rate of 6 dB per doubling of distance in the near field while multipole speakers may attenuate at a rate of 12 dB, 18 dB or more per doubling of distance.
The simplest multipole loudspeaker is the dipole loudspeaker. This type of speaker exhibits a xe2x80x98figure eightxe2x80x99 sound directivity pattern consisting of first and second sound pressure lobes extending outward from and substantially in opposite directions from the speaker means. Dipoles also exhibit a null zone lying in a plane perpendicular to a central longitudinal axis of the first and second sound pressure lobes. The directional capabilities of this type of loudspeaker allow it to be oriented such that the main sound pressure lobes are directed toward a user and away form third parties. This provides the user with enhanced privacy.
In general, multipole loudspeakers may be supported in a relative position to an intended listener to direct sound into a specific spatial region conveniently located for alignment with the intended listener""s ear. For convenience, the specific spatial region on each side of the loudspeaker is considered in the terminology used hereinafter to be in the form of a sound pressure lobe with a particular directivity pattern. Using a multipole speaker which has a null zone also finds application in wearable handsfree devices, for example, as the speaker can be oriented such that in operation one sound pressure lobe may be directed toward the user""s ear, the other lobe directed downward into the shoulder or chest area of the user while the null zone extends laterally away from the user in the direction of third parties. In this case, privacy is a direct consequence of the null plane conveniently extending laterally away from the user in the direction of third parties.
Providing multipole loudspeakers in communication devices which require speakers provides for a less intrusive environment as these loudspeakers are better able to direct reproduced sound in the direction of the user and away from unintended parties. As well, the user of a speaker telephone which incorporates a multipole loudspeaker, for example, will benefit as he or she will be able to listen to a caller or voice mail messages in a handsfree mode with a greater degree of privacy. Multipole loudspeakers may also be used in other personal handsfree communications devices such as terminals or personal computers etc. with the loudspeakers oriented to direct sound into the specific spatial region within which a user""s ear would be located.
In applications such as the automotive cellular industry, the use of multipole speakers to reproduce a received voice conversation would provide a similar degree of privacy to a user of a cellular terminal when the terminal is operated in the hands free mode. Specifically, multipole speakers could be supported in a relative position to a user to direct sound into a specific spatial region conveniently located for alignment with, for example, the user""s ear with the null planes or smaller sound pressure lobes directed in the general direction of the other seating positions within the automobile. The multipole speakers could be supported in or on a seat head-rest, be supported from the ceiling or even be supported by the door frame assembly.
In all of the above applications, the user is able to directly take advantage of the directionality capabilities of the multipole loudspeaker. In particular, a dipole loudspeaker implementation uses the standard null in its sound directivity pattern to provide a measure of privacy as the unintended listener may most often be assumed to be aligned with the standard null plane. For the most part, this is a valid assumption. However, the privacy afforded by such dipole loudspeaker implementations will be compromised if the unintended listener is, in fact, not aligned with the standard null surface of a dipole implementation.
The present invention addresses applications where the direction in which the reduction of sound transmission is known, and may not be in the null of a dipole loudspeaker implementation. Using a dipole loudspeaker with two acoustic sources and introducing a pre-determined phase difference between the signals to these two sources, the null can be moved to any specified direction. In addition, a microphone can be placed in the direction of the desired null, and a feedback mechanism can be used to align the null with the direction of minimum desired transmission.
This invention would be particularly effective in applications of a multipole speaker which may involve embedding the implementation into a headrest, a seat, or other object where the direction of minimum transmission is known. Advantageously, the invention uses presently available commercial components and, as such, its implementation should not add much cost to an overall system.